The present invention relates to a magnetometer. It is used in the precise measurement of weak magnetic fields (typically in the range 20 to 70 .mu.T corresponding to the values of the earth's magnetic field).
The magnetometer according to the invention is of the so-called resonance magnetometer type and a general description thereof is provided in the article by F. HARTMAN entitled "Resonance Magnetometers", published in the journal "IEEE Transactions of Magnetics", vol. MAG-8, No. 1, March 1972, pp 66 to 75.
A resonance magnetometer is an apparatus which, immersed in a magnetic field Bo, supplies an electrical signal of frequency F, whose value is linked with Bo by the so-called LARMOR relation: EQU F=.gamma.Bo
in which .gamma. is the gyromagnetic ratio (of an electron or nucleon as a function of the substance used). For example, for the electron, said ratio is 28 Hz/nT.
In the case of such equipment, the optical pumping magnetometer occupies a privileged position. The general construction of a magnetic resonance, optical pumping magnetometer is diagrammatically shown in FIG. 1.
An at least partly transparent cell 10 is filled with a gas 12, generally helium at a pressure of 1 to a few Torr. A light source 14 supplied a light beam 18, whose wavelength is approximately 1.1 .mu.m in the case of helium. This beam is appropriately polarized by a means 16 and then injected into the cell 10.
In addition, a so-called weak or gentle radiofrequency or high frequency discharge is produced in the gas by a generator 30 connected to two electrodes 32, 33 arranged around the cell 10. This discharge produces atoms in a metastable state (2.sup.3 S.sub.1 in the case of helium). The incident light beam 18 "pumps" these atoms from the metastable state to bring them into another excited state (2.sup.3 P).
In the presence of a magnetic field Bo, the energy levels are subdivided into sublevels, called ZEEMAN sublevels. A resonance between such sublevels can be established by a high frequency field (magnetic resonance) or by a modulation of the light (double optical resonance; COHEN, TANNOUDJI, Ann. Phys., 7, 1962, p 423). In the case of isotope 4 helium, the resonance is established between two electronic ZEEMAN sublevels of the metastable state. This resonance is revealed by various known electronic means, whereof one variant is shown in FIG. 1. It is a winding 20 positioned on either side of the cell 10 (in a so-called HELMHOLTZ arrangement), a high frequency generator 22 and a photodetector 24 receiving the light radiation which has passed through the cell, an amplifier 25, a synchronous detection means 21 and an integrator 23. All these means 21 to 26 will be referred to hereinafter by the reference CC. The generator 22 supplies the winding 20 with current at the frequency F, which creates an oscillating magnetic field, whereof one component maintains the resonance and on return modulates the light beam which is passed through the cell, said modulation constituting the signal. It is revealed by the synchronous detection at the output of the photodetector, via the amplifier. The reference is given by the generator. The output of the synchronous detection means corresponding to the component of the signal in phase with the reference serves as an error signal and the integrator eliminates its static error. This error signal readjusts the frequency F of the synthesizer to the LARMOR frequency. For this purpose the synthesizer must be voltage-controllable and it can also be replaced by a voltage-controlled oscillator (V.C.O.).
Thus, an electric resonance signal is established in said loop at the LARMOR frequency. A frequency meter 26 gives it the value F. The field to be measured Bo is deduced by the relation Bo=F/.gamma.. Helium magnetometers of this type firstly used helium lamps. The recent availability of lanthanum-neodymium aluminate (or LNA) crystals has made it possible to produce lasers tunable about the wavelength of 1.083 .mu.m precisely corresponding to the optical pumping line of helium. Therefore this type of laser has naturally taken the place of these lamps and has led to a significant performance improvement, so that interest has been reawakened in such equipment. Such a magnetometer equipped with a LNA laser is described in FR-A-2 598 518.
Although satisfactory in certain respects, such magnetometers still suffer from disadvantages. Thus, by their very nature, they are highly anisotropic, both in amplitude and frequency. Signal suppressions occur for certain orientations of the magnetic field to be measured. These unfavourable orientation correspond either to certain propagation directions of the light beam (in the case of a circular polarization), or to certain polarization directions (in the case of a linear polarization). Optical pumping then no longer produces the requisite polarization of the ZEEMAN sublevels of the atoms, or the detection of the resonance proves to be ineffective.
Various solutions have been proposed for obviating this disadvantage. For example, the US company Texas Instruments recommends the use of several cells oriented in such a way that at least one supplies a usable signal. The Canadian company Canadian Aviation Electronics recommends orienting the magnetometer in an appropriate manner with respect to the field to be measured.
As the suppression zones of the signal are more extensive for a linearly polarized beam than for a circularly polarized beam, preference is generally given to working with circular polarization. However, with this type of polarization, a frequency shift phenomenon occurs due to the optical pumping and this gives rise to measurement errors.
Texas Instruments obviates this disadvantage by doubling the number of cells and by making one of them operate with clockwise circular polarization and the other with anticlockwise circular polarization. The frequency shifts observed in the two cells then have opposite signs and a compensation is possible by forming the mean of the two measured frequencies.
All these solutions, which amount to increasing the number of apparatuses, are not very satisfactory, due to excessive overall dimensions, the need to balance the various measuring channels, the control of the orientation of the cells, high power consumption, etc. Moreover, any installation must be produced in an amagnetic environment, which causes serious technological problems.